I thought that one of the problems with the Welkin was its wing section. It had a very long span but relatively thick wing.
Consequently at extreme altitude the gap between cruise and stall speed (coffin corner) was minimal even by the standards of the day.
If true a strange error in a plane specifically designed to fight at extreme altitude.
And the high altitude Mosquito cut off its reason for existence ... with the same tactic as Aboukir used with the Ju86 ... pepper the pressure cabin with light MG fire and hit it if necessary when it descended (it wasn't necessary). Losses of Ju86s dissuaded the Luftwaffe from trying for extreme altitude as the RAF knew how to handle this. Hence no demand for the Welkin.